1. Field
The following description relates to a light emitting diode (LED) driving circuit for backlight, and more particularly, to a circuit and a method for generating a pulse width modulation (PWM) signal for a DC-DC converter using a dimming signal, and a LED driving circuit for backlight having the same.
2. Description of Related Art
A liquid crystal display (LCD), which is a representative flat panel display apparatus, displays an image using an electric and optical characteristic of liquid crystal. The LCD is being widely used because it has many advantages compared to other display apparatuses, such having a slim thickness, being lightweight, having low power consumption, and having low driving voltage. However, since an LCD panel used in the LCD is a non-emissive element that is unable to emit light by itself, the LCD requires an extra backlight unit in order to supply light to the LCD panel.
A cold cathode fluorescent lamp (CCFL) and a light emitting diode (LED) are used as such an extra backlight unit. Since the backlight unit using the CCFL uses mercury, the backlight unit may cause an environmental pollution problem and have a low speed response. In addition, the backlight unit of the CCFL has the demerits of poor color reproduction and generation of pre-set white light.
On the other hand, the backlight unit using the LED does not use a material harmful to the environment and is capable of achieving a high-speed response and impulsive driving. In addition, the LED backlight unit has the merit of good color reproduction and an ability to adjust color coordinates and brightness of light by adjusting amounts of light of red, blue, and green LEDs. The LED light unit creates white light by mixing red light, blue light, and green light appropriately. Therefore, the LED backlight unit includes a plurality of red LED arrays for emitting red light, a plurality of blue LED arrays for emitting blue light, and a plurality of green LED arrays for emitting green light.
The LED backlight unit adjusts brightness of the LED using a dimming method. Dimming methods include an analog dimming method and a digital dimming method. The analog dimming method adjusts the brightness of the LED by adjusting an amount of electric current supplied to each of the LEDs. In other words, according to the analog dimming method, if the amount of electric current to each LED is reduced by half, the brightness of each LED is reduced by half. A pulse width modulation (PWM) dimming method, which is the digital dimming method, adjusts brightness of the LED by adjusting a ratio of on-off time of each LED according to a PWM signal. For example, if a PWM signal having an on-off time ratio of 4:1 is provided to each LED, brightness of the LED reaches 80% of maximum brightness.
In order to adjust the brightness of the LED in the above-described digital dimming method, a clock signal of a DC-DC converter for supplying power to the LED and a dimming signal for adjusting an amount of electric current in the LED are separately provided. In general, the frequency of the clock signal of the DC-DC converter is relatively long and the frequency of the dimming signal is relatively short, and the clock signal of the DC-DC converter and the dimming signal are not synchronized with each other. As an on-period of the dimming signal becomes shorter, it is more difficult for the DC-DC converter to maintain sufficient output voltage to drive the LED as much as is desired.
FIG. 1 is a view illustrating an example of waveforms to explain an operation of generating a PWM signal based on a dimming signal in the related art. Referring to FIG. 1, “CK” indicates a clock signal of a DC-DC converter. “DM_H” and “DM_L” indicate dimming signals. “DM H” is a dimming signal of a relatively long on-period, and “DM_L” is a dimming signal of a relatively short on-period. “PWM_H” and “PWM_L” indicate PWM signals provided to the DC-DC converter. “PWM_H” is a PWM signal obtained based on the dimming signal (DM_H), and “PWM_L” is a PWM signal obtained based on the dimming signal (DM_L).
In the case of the dimming signal (DM_H) having the long on-period, a plurality of PWM signals (PWM_H) are generated during the on-period and are provided to the DC-DC converter, so that the DC-DC converter maintains stable output voltage. However, in the case of the dimming signal (DM_L) having the short on-period, no PWM signal (PWM_L) is generated during the on-period. In other words, since no PWM signal is generated during one period (1T (DM)) of the dimming signal, the DC-DC converter cannot maintain stable output voltage.